This invention relates to a navigation system for motor vehicles for guiding a driver when the vehicle approaches an intersection on a predetermined route to be taken. More articularly, this invention relates to such a navigation system where guidance is provided according to the current velocity of the vehicle and misleading guidance due to erroneous detection of vehicle position is prevented.
A conventional navigation system for vehicles is provided with a screen such as cathode ray tube CRT, a liquid crystal display, or other display means for displaying thereon a road map. The route and current position of the vehicle are indicated on the displayed road map. When a vehicle approaches an intersection on a predetermined route, the driver is informed by an indication on the screen or by an audible message.
Such a navigation system continually detects the current position of the vehicle and obtains the distance between the current vehicle position and the next intersection on a predetermined route. When the distance becomes a given value or less, the system informs the driver that the vehicle is approaching the next intersection. Therefore, if the current position as detected is incorrect due to a detection error, the system may wrongly inform the driver that the vehicle is approaching the intersection when the vehicle actually is not approaching the intersection.
In this case, since the driver associates the announced information with the route indicated on the displayed road map, the direction he perceives the vehicle is heading for may be incorrect. Resultantly, the driver may turn right at the intersection where he should actually go straight, or go straight at the intersection where he should turn right, etc.
There is known another type of navigation system by which a driver is informed by an audible message of the direction to take at the intersection the vehicle is approaching. In this type of system, the driver takes, at the intersection, the direction as informed without paying attention to the route indicated on the road map. As a result, even if the vehicle is deviating from a predetermined route, the driver can recognize the deviation only long after it takes place, resulting in a large deviation from the predetermined route.
Further, in these prior art navigation systems, the value of the distance between the current vehicle position and a target intersection that triggers guidance of the driver at an intersection is preset at a constant value and is not varied according to the velocity of the vehicle. Therefore, regardless of the vehicle velocity, the time period that the driver has in preparation for the target intersection is constant, and the driver tends to feel uneasy because the time period is too long when driving at a low speed or too short when driving at a high speed.